Numerous types of foods, including beverages, are packed in containers, including cans, glass bottles, or polyethylene terephthalate (PET) bottles. Many times, the foods need to be heated for cooking and/or need to be pasteurized, and the insides of the containers need to be sterilized. Various methods have been developed to reduce or eliminate the need for preservatives to preserve foods and increase shelf life.
Hot-fill sterilization of containers, such as bottles containing beverages, is a well-known method for sterilizing containers to render the container essentially free of microorganisms and microbial growth. Typically, hot-fill sterilization is achieved by heating a liquid to a temperature of at least about 185 degrees Fahrenheit (85 degrees Celsius) using a thermal process, for example, electrical resistance heating or a heat exchanger arrangement. After this heating of the liquid, the heated liquid is placed in a holding tube to maintain the temperature of the fluid for a sufficient amount of time so that the liquid is sterilized from commonly found microorganisms. The heated liquid is then filled into containers. Prior to cooling the containers, the containers are often manipulated to ensure that the hot liquid comes into contact with all of the interior surfaces of the containers, including any container closures (e.g., caps). Such manipulation generally involves tilting or inverting the containers to contact the hot liquid with all of the interior container surfaces for a time sufficient to sterilize the inside of the containers and their respective closures. Once the containers have been completely sterilized, they may then be cooled and safely stored as a shelf stable product without refrigeration, usually for at least three months.
There are drawbacks to conventional hot-fill sterilization. Certain liquids, for instance beverages, contain solid inclusions (e.g., particles) as well as liquid. Inclusions, however, may agglomerate within small portions of a container, such as the neck and/or cap of a bottle. The agglomeration of inclusions may inhibit hot liquid from reaching those portions of the container during manipulation, and result in a container that is not completely sterilized. For instance, one inversion method that has shown to result in agglomeration of inclusions in the neck is the so-called “laydown method.” The laydown method involves tilting a hot-filled bottle from a vertical standing position to a horizontal lie-down position, over a time period of about 1-2 seconds. The bottle remains in this position for a specified amount of time, and then is raised back to a vertical standing position. Another inversion method that has shown agglomeration of inclusions in the neck is the so-called “camel hump inverter.” The camel hump inverter method involves grabbing a vertical upright bottle by rubber grippers and tilting the bottle until it is tipped 90 degrees on its side. The bottle is transferred to an upright position when it reaches the top of the inverter, and then is tilted 90 degrees on its side in the opposite direction. Accordingly, it would be desirable to prevent agglomeration of inclusions in portions of a container to ensure proper sterilization of the interior of the container.
Another sterilization method is sometimes referred to as tunnel sterilization or tunnel pasteurization. Conventional tunnel sterilization typically involves filing a container, such as a bottle, with a food, such as beverage, and then closing the container, such as capping a bottle. Containers or bottles are loaded at one end of a tunnel and passed under sprays of hot water as they move along a conveyor. The sprays are arranged so that the bottles are subjected to hot water until the pasteurization temperature of the beverage is reached. This also has the effect of sterilizing the container. The bottles are then cooled with sprays of cool water until they are discharged from the end of the tunnel. Conventional tunnel sterilization typically involves use of a fuel-burning boiler to produce steam, the steam is then cooled to produce hot water, and the hot water is then sprayed over the capped bottles as they move along the conveyor. Conventional tunnel sterilization has an energy efficiency of about 30-50%, i.e., about 30-50% of the amount of heat (which may be expressed in British Thermal Units, i.e., BTU) generated by a fuel-burning boiler is actually delivered into the product. Conventional tunnel sterilization also typically requires 10 minutes or more, and frequently at least 20 minutes or more to achieve sufficient pasteurization of the beverage and sterilization of the interior surfaces of the container. For example, conventional tunnel sterilization typically requires the product be heated with sprayed hot water for about 10-12 minutes in “a come up” zone of the tunnel until the product reaches a temperature of about 160 degrees Fahrenheit, and then be held at that temperature for about another 10 minutes at this target temperature. The longer a product and container are subjected to high temperatures, the greater the risk of thermal abuse of the product and the container, leading to a greater risk of adverse taste, and degradation of the product and the container. Conventional tunnel sterilization may not be possible for certain products and/or containers, or may require an increase in the thickness of containers, e.g., increasing the thickness of PET bottles to achieve sterilization of PET bottles filled with beer to ensure a sufficiently strong bottle after sterilization.
Aseptic sterilization is another way to sterilize the interior surfaces of containers. In a typical aseptic sterilization, a container is sterilized with an aqueous solution of hydrogen peroxide (H2O2) to achieve a germ-killing effect, and a pasteurized product is then filled into the sterilized bottle. The pasteurized product is typically heated using a thermal heating process, and held at a pasteurization temperature for a sufficient period of time e.g., in a centralized tank and/or holding tube (similar to that used in hot-fill sterilization), then allowed to cool prior to being placed in a sterilized bottle. The equipment used for aseptic sterilization typically costs many millions of dollars and is much more expensive than equipment for tunnel sterilization. Changing a production line from a hot-fill sterilization or tunnel sterilization to aseptic sterilization entails high conversion costs.
The above methods typically have a large carbon footprint in that they typically require heating of product using a thermal process, for example, heating the product with electrical resistance heating or a heat exchanger arrangement (such as for conventional hot-fill and aseptic sterilization), or in the case of conventional tunnel sterilization, heating the product with hot water, which is generated from steam from a fuel burning boiler. It would be desirable to achieve sterilization of product and the interior surfaces of containers filled with product that has a lower carbon footprint and is more energy efficient than sterilization methods that use thermal process heating. It would be also be desirable to achieve sterilization of product and the interior surfaces of containers filled with product using methods that increase production rates over conventional methods.
Microwave energy or radiation has been used to heat product to provide longer shelf life, thus enabling central preparation of products for shipping. Commercial food preparation and packaging processors have found it difficult, however, to use microwave energy to increase production rates.
Various methods for increasing heating rate with intent of increasing production rates are known. One known method is to use pulse microwave energy radiation; another is simultaneous use of multiple sources of microwave energy, such as, for example, irradiation from several directions. In these various methods, microwave energy has been used both before and after product is packaged. However, containers often are damaged because a local product temperature exceeds the service temperature of the container, and organoleptic properties and characteristics of the product often are degraded by long periods of exposure to a locally high temperature in the product.
Thus, none of these known methods using microwave energy is satisfactory. Known methods using microwave energy result in unevenly-heated product that do not ensure sufficient shelf life. Known methods using microwave energy also have not been successful at significantly shortening processing time, and organoleptic properties and characteristics of product often are degraded, and containers often are damaged.
Therefore, there exists a need for a method for sterilization of product, especially food product, which reduces processing time without damaging containers and without degrading organoleptic properties and characteristics of the product, yet does not have the disadvantages of prior methods. It would also be desirable to achieve sterilization of product and interior surfaces of containers for products and containers not achievable using conventional methods and/or at a lower cost than conventional methods.